Fluoropolymers are primarily produced via heterogeneous polymerization reactions, including suspension, emulsion and microemulsion systems. Generally, each of these reactions requires at least one monomer and a radical initiator in a suitable reaction medium. In addition, emulsion polymerizations of halogen-containing monomers generally require a surfactant capable of emulsifying both the reactants and the reaction products for the duration of the polymerization reaction. The surfactant of choice in the synthesis of fluoropolymers is generally a perfluoroalkyl surfactant. The high degree of fluorination of the surfactant avoids atom transfer between a growing polymer chain and the surfactant during polymerization, which will result in lowered molecular weights in the product and likely inhibition of the reaction. The most common perfluoroalkyl surfactant in the production of halogenated polymers is ammonium perfluorooctanoate (AFPO).
The emulsion polymerization of vinylidene fluoride (VF2) using a fluorinated surfactant and isopropyl peroxydicarbonate (IPP) as a free-radical initiator, is taught in U.S. Pat. No. 3,475,396. This polymerization process has since been refined to provide polymers having various improved properties. See U.S. Pat. Nos. 3,857,827; 4,076,929; 4,360,652; 4,569,978; 6,187,885.
Fluorosurfactants are expensive, specialized materials, however. In addition, because of their high stability, they tend to persist in the environment. Because of their resistance to chemical degradation, fluoroalkyl surfactants have the potential to accumulate in the environment and in organisms.
New polymerization processes are needed that utilize non-perfluoroalkyl surfactants or reduced amounts of perfluoroalkyl surfactants. In order to address this issue, several different approaches have attempted to reduce or eliminate the use of perfluoroalkyl surfactants in the polymerization of halogen-containing monomers.
Some emulsion polymerization processes have been demonstrated which employ partially fluorinated surfactants instead of perfluorinated surfactants. See U.S. Pat. Nos. 4,524,197; 5,763,552. Another attempt to reduce the amount of perfluoroalkyl surfactant in heterogeneous polymerization involved a protocol wherein a conventional fluorinated surfactant was added in combination with a non-fluorinated hydrocarbon surfactant. However, this modification served to substantially lower the rate of the reaction. See, WO 95-08598A, the entire disclosure of which is incorporated herein by reference.
Another attempt to reduce the amount of perfluoroalkyl surfactant in heterogeneous polymerization involved a protocol wherein a conventional fluorinated surfactant was added in combination with a non-fluorinated hydrocarbon surfactant. However, this modification served to substantially lower the rate of the reaction. See, WO 95-08598A, the entire disclosure of which is incorporated herein by reference.
U.S. Pat. No. 2,559,752 assigned to E. I. du Pont de Nemours relates to an “Aqueous colloidal dispersions of polymers”. Stable aqueous colloidal dispersions of polymerized ethylenically unsaturated organic compounds are obtained by carrying out the polymerization in the presence of a H2O-soluble polymerization initiator, such as an alkali persulfate or an aliphatic azo compound (cf. U.S. Pat. No. 2,471,959, C. A. 43, 6002 g), and a polyfluorinated ionizable dispersing agent (I). The I is taken from various groups of compounds including:
polyfluoroalkanoic acids, X(CF2)nCO2H, polyfluoroalkyl di-H phosphates, X(CF2)nCH2OPO(OH)2 [obtainable from X(CF2)nCH2OH and P2O5 or POCl3), and their NH4 or alkali-metal salts];
polyfluoroalkyl H sulfates, X(CF2)nCH2OSO3H and their NH4 or alkali-metal salts;
polyfluoro alkane phosphonic acids, H(CF2)nPO(OH)2 [obtainable from C2F4 and a dialkyl phosphite in the presence of a free-radical producing catalyst, followed by hydrolysis] and their NH4 or Na salts.
U.S. Pat. No. 6,869,997, incorporated herein by reference, describes the use of a 3-allyloxy-2-hydroxy-1-propanesulfonic acid salt as the surfactant in the preparation of a fluoropolymer.
U.S. Pat. No. 6,841,616 incorporated herein by reference, describes the use of a siloxane, based surfactant as the surfactant in the preparation of a fluoropolymer.
None of the art references described above mentions the use of non-fluorinated alkylphosphonic acids, or their salts, as surfactants in the synthesis of fluorinated polymers.
Surprisingly it was found that alkyl phosphonic acids or their salts may be used as surfactants in the synthesis of fluorinated polymers, and thereby eliminate or significantly reduce the use of fluorinated surfactants.